This invention is particularly suited as a coating material for electrical connectors. More specifically, a composite material containing a ductile metal and a polymer is provided. The coating reduces the frictional force generated within an electrical connector minimizing insertion and fretting wear.
Electrical interconnection between two devices may be achieved by inserting an insertion component such as a pin or blade into a mating socket. Both the insertion component and the socket are manufactured from a material having low bulk resistivity such as copper or a copper alloy. The socket is usually imparted with a spring temper to exert a compressive force. The compressive force keeps the insertion component in place and decreases contact resistance between the socket and insertion component.
The selection of a copper alloy for the components of an electrical connector minimizes bulk resistivity. Contact resistance, the resistance (in ohms) between the mated components of a connector, is equally as important. Contact resistance is dependent on bulk resistivity and the interface between the insertion component and the socket.
On a microscopic scale, this interface is comprised of a number of contact spots. The contact resistance is dependent on the number of conductive contact spots (known as "a-spots") and the area of each conductive spot. The larger the aggregate area of a-spots, the lower the contact resistance. The number and area of contact spots is variable. Over time, oxidation may reduce the area of the a-spots. Further degradation may occur due to wear resulting in the accumulation of debris within the contact region to reduce the conductive contact area. As the total area of a-spots decreases, contact resistance increases. The increasing contact resistance leads to an increasing voltage drop across the electrical connector. When the voltage drop becomes too severe, connector failure results.
Copper and copper alloys readily oxidize at elevated temperatures and will gradually oxidize at room temperature. Certain copper alloys are relatively oxide resistant at low temperature operations and suitable for certain electrical applications without a coating. For example, in high voltage, room temperature applications such as household appliances, electrical requirements are satisfied by uncoated copper alloy C260 (cartridge brass having the nominal composition 70% copper/30% zinc). For lower voltage applications where precise measurements are required or where elevated temperatures may be experienced, the connector and socket are coated with an oxidation resistant metal such as tin, gold and lead/tin alloys. A thin coating, on the order of 1-4 microns, is applied to the connector by a process such as electrolytic deposition, electroless deposition or, for low melting metals and alloys, by hot dipping.
Each coating has certain inherent limitations. Gold is a soft metal and does not oxidize. Gold alloys are harder and generally oxide resistant. The high cost of the metal limits its use as a coating to those connectors requiring extremely high reliability.
Tin and tin/lead alloy coatings form surface oxide layers. The coatings distort during insertion. The thin oxide layer is fractured in the contact region and a fresh, unoxidized surface makes contact within the electrical connector. Repeated insertion and removal of the connector will lead to insertion wear. The soft metal coating gradually erodes, exposing the copper alloy connector substrate. The exposed substrate will oxidize increasing contact resistance.
Even if the insertion component is not repeatedly inserted and removed from the socket, wear is a problem. Fretting corrosion may occur. Fretting corrosion is surface damage usually manifest in an oxidizing environment when two surfaces, one or both of which are metals, are in close contact, under pressure, and subject to slight relative motion. Connector assemblies in automotive and industrial applications are often subject to some form of vibration. Thermal expansion and contraction of the connector components also causes minute relative movement between the mated contacts. Each small movement exposes fresh metal. As the fresh metal oxidizes, oxide debris accumulates. The slight fretting motion, on the order of a few microns, is not sufficient to remove the debris from the contact area. The debris, which contains tin oxide (SnO.sub.2), increases the contact resistance of the connector.
One method to minimize insertion wear and to reduce fretting corrosion is to deposit an external thixotropic lubricant. One lubricant which has been used for many years on rotary dials is a grease composed of a mixture of 2-diethyl hexyl sebacate and high molecular weight methacrylate polymer. The lubricants extend the lifetime of the metal connector coating and do not adversely effect the contact resistivity. Externally applied coatings are subject to misapplication by the connector assembler, possibly contaminating other electronic components. The subsequent evaporation of the lubricant and the entrapment of dust both may lead to an increase in the contact resistance.
Inorganic, nonmetallic particulate materials have been blended into coatings to increase hardness and wear resistance and to reduce friction. Among the composite coatings of tin are mixtures with silicon carbide, aluminum oxide, tungsten carbide and molybdenum disulfide. Composite coatings formed from a mixture of tin and the first three additives are harder than pure tin and tend to have high contact resistance. Composites of tin and molybdenum disulfide while relatively soft, do not maintain a low contact resistance under fretting conditions.
A composite of tin and an organic particulate has been applied to decrease frictional wear in bearings. U.S. Pat. No. 4,665,113 discloses a joint agglomeration of granular polytetrafluoroethylene (PTFE) and a metal containing filler selected to be copper, tin or their alloys. The agglomeration is then molded into slide bearings. PTFE is known by the trademark TEFLON.TM. (DuPont Corporation, Wilmington, Del.). The filler occupies from 1 to 75 percent by weight of the agglomeration.
U.S. Pat. No. 4,312,772 discloses another bearing material. A porous layer of copper or a copper base alloy is formed on a steel backing sheet. The porous layer is impregnated with a mixture of lead fluoride, lead or lead/tin alloy, and PTFE. Depending on the application, additional additives such as molybdenum, tungsten disulfide, cadmium oxide, aluminum oxide, calcium fluoride, lithium fluoride, graphite, lead iodide, glass fibers, carbon fibers and a phosphate may be added to the mixture.
Bearing materials are designed to minimize friction and maximize durability. Electrical requirements are not a consideration. The bearing materials have a relatively high concentration of PTFE and are unsuitable as coatings for electrical contacts. The Applicants have discovered that within a limited compositional range and within a limited polymer particle size range a coating material for electrical contacts from a composite of tin and PTFE may be formed. The composite coating has excellent electrical characteristics and improved wear properties.